With the recent developments in the enzymology of the complex vitamin D3 endocrine system, it should be possible to elaborate upon the regulation of this endocrine system. The systematic approach to the study will involve the isolation, characterization and evaluation of the biochemical parameters of the vitamin D3-25-hydroxylase, 25-hydroxy-vitamin-D3-1-hydroxylase and the 25-hydroxyvitamin-D3-24-hydroxylase. Additionally, the mode of the production of the new yet structurally undefined renal microsomal metabolites of vitamin D3 will be investigated. Fluorescent antibodies against specific components of the various hydroxylases will be prepared to ascertain the molecular localization of these enzymes and their components within the secretory organs. These studies will form the basis for investigation of the regulation of the integrated endocrine system associated with vitamin D3 metabolism.